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Full text of "Text Book Of Mechanical Engineering"

366

Diagram of Work done.

ils!

similarly resisted by the purely tensile force Ft.    A force diagram
being drawn for each case,

Fc - fi Fs    and    Ft = ,/JTF,

11

Sill

Nature of Tensile and Compressive Stresses.  When
a plain tin-notched bar is broken by pulling, lines of cleavage
appear on the surface, inclined at 45 to the axis ; and the final
fracture is cup-shaped. Compression fractures are also inclined
at 45 and are often wedge-shaped. The evident deduction is
that rupture takes place on shear planes in both cases, and that
the three simple stresses are interdependent.

Work done by Uniform Forces.  The unit of work is
a foot-pound, or one pound exerted through a distance of one foot.
One pound acting through two feet, or two pounds through one
foot, are each two foot-pounds. Hence :

Work = pressure x distance

= pounds  x feet = foot-pounds.

These forming a product may be represented by an area, for
length x breadth == area, and A, Fig. 325, is therefore the diagram
of work with uniform force :

Work done = pounds x feet ~ o x x o Y ~ area A.

Work done by Variable Forces is shewn by diagram atB,
Fig. 325, As the body moves from ox to 5, the pressure varies
as GJ xp 2 &, Sic. Now, work done between o'j and i can neither
be ox KX x i ft nor i a x i ft, but must be the average of these,
or O-L /x i. In like manner the other dotted rectangles shew the
work between the remaining intervals, and their addition,
Area ox xx b vx  work done.

Work done in Deformii^g a Bar is found at r, Fig. 326.
Divide o B into ten parts, and erect pei^endiculars between the
divisions.   Measure the vertical ordinates in tons, then
Total of orflnates ^ ^ ^ .

10                                        '

men load x extension*^ work in inch tons. ($ee#. 1065.)